First draft of docs for nancallback

doc/nancallback.md

@@ -0,0 +1,52 @@
+#Callback
+`Callback` makes it easier to use `v8::Function` handles as callbacks.
+
+##Callback
+A class that wraps a `v8::Function` handle, protecting it from garbage collection.
+```c++
+class Callback {
+ public:
+  Callback();
+
+  explicit Callback(const v8::Handle<v8::Function> &fn);
+
+  ~Callback();
+
+  bool operator==(const Callback &other) const;
+
+  bool operator!=(const Callback &other) const;
+
+  NAN_INLINE
+  v8::Local<v8::Function> operator*() const;
+
+  v8::Local<v8::Value> operator()(
+      v8::Handle<v8::Object> target
+    , int argc = 0
+    , v8::Handle<v8::Value> argv[] = 0) const;
+
+  v8::Local<v8::Value> operator()(
+      int argc = 0
+    , v8::Handle<v8::Value> argv[] = 0) const;
+
+  void SetFunction(const v8::Handle<v8::Function> &fn);
+
+  v8::Local<v8::Function> GetFunction() const;
+
+  bool IsEmpty() const;
+
+  v8::Local<v8::Value>
+  Call(v8::Handle<v8::Object> target
+     , int argc
+     , v8::Handle<v8::Value> argv[]) const;
+
+  v8::Local<v8::Value>
+  Call(int argc, v8::Handle<v8::Value> argv[]) const;
+};
+```
+
+###Example
+```c++
+v8::Local<v8::Function> function;
+Callback callback(function);
+callback->Call(0, 0);
+```